A wealth of research indicates that children's social play contributes to the development of social, cognitive, and motoric skills. However, what causes children to collaborate with one another is not well understood. This research seeks to identify developmental and proximate factors that cause young children to collaborate. In doing so, the proposal applies a bio- ecological approach in line with an interdisciplinary trend in the fields of developmental psychology and biology. Specifically, prior research indicates that the early home environment and experiences (e.g. parental care, stressful and/or chaotic experiences) impact children's developing arousal systems (e.g. HPA axis activity). Importantly, children's arousal systems support their ability to process information for learning and social interactions. A large body of research indicates that a moderate arousal response to cognitive and social challenges summons the physiological resources needed to address challenges. However, hypo-arousal (i.e. sluggish, unmotivated) and hyper-arousal (i.e. distractible, hypersensitive) states are associated with poorer cognitive and social outcomes. The current project applies this robust body of research to evaluating predictors of children's collaboration. Specifically, it investigates whether children's early environment and experiences shape their arousal and social cognition to support, or not, children's ability to collaborate with a peer in te face of challenge. In doing so, the proposal measures environmental, physiological, and social-cognitive factors that have previously proven significant for children's development. In order to obtain variation in the environmental variables and seek a sample that better reflects the population at large, half of the dyads of children will be recruited from low income backgrounds. The experimental paradigm employed is a toy construction activity developed and piloted by the applicant. Children build toys step-by-step with colorful boxes, similar to a large Lego set. The steps vary widely in difficulty, enabling us to observe and analyze how children respond to challenges. Importantly, this experimental paradigm permits children to choose when and how much to coordinate with their peer partner, thereby providing a graded measure of when and how much each dyad collaborates. The analyses will assess a) which cognitive, physiological, and social-cognitive factors relate to when and how much children collaborate and b) whether environmental factors relate to the factors that predict children's collaboration. In this way, the proposal will further our understanding of a) what factors contribute to children's collaboration and b) how children's early environment may relate to their social and cognitive development.